The invention relates to a presentation system comprising an image projection screen and an overhead projector which is provided with an illumination system, a support for supporting an image record carrier and a head for projecting the image present in the image record carrier on the image projection screen.
The image record carrier may be a transparent plate referred to as overhead sheet, which has a permanent image, or an image display panel comprising a matrix of pixels with which images can be generated electronically. Such an image display panel may be connected, for example to a computer so that computer images can be displayed. The panel may be, for example a TN-LCD panel (twisted-nematic liquid crystal display panel) so that the direction of polarization of the incident light is modulated in conformity with the image information to be displayed. The image display panel may alternatively be a PDLC panel (polymer-dispersed liquid crystal panel) which operates on the basis of dispersion of the incident light, either or not in combination with two complementary polarizers. An image of the image information present in the image record carrier is formed and projected by means of the projection head of the overhead projector.
The invention also relates to an overhead projector which supplies light having a well-defined state of polarization for use in such a presentation system.
An overhead projector suitable for use in a presentation system of the type described in the opening paragraph and intended for projecting the image of a transparent foil is known from, for example U.S. Pat. No. 3,126,786. The overhead projector described in this Patent successively comprises an illumination system consisting of a lamp and a reflector, a condenser lens, preferably in the form of a Fresnel lens, a support for supporting an image record carrier and a projection head for reflecting and focusing the projection beam from the image record carrier on a projection screen.
A drawback of a presentation system with an overhead projector as is known from said Patent is that, in addition to the light coming from the overhead projector and comprising the image information, which light is further referred to as signal light, also ambient light will be incident on the projection screen and reflected by this screen towards the audience. To obtain an image on the projection screen which is sufficiently rich in contrast during a presentation, at which illustrations, graphs and the like are shown by means of an overhead projector, the audience space will have to be darkened considerably. This darkening, which is often necessary more than once during a lecture, is a disturbing and detracting operation. Moreover, the darkening reduces the quantity of light for the audience. This may be troublesome if they want to make notes, for example. Moreover, the audience is less well visible to the speaker, due to the darker ambience. Until now, the darker ambience has been accepted as a necessity and no useful overhead presentation system has been conceived which can work with a satisfactory contrast without darkening. An increase of the power of the lamp in the illumination system, so that the signal light will be considerably more intensive than the ambient light, is not a good solution because the lamp must then be cooled thoroughly, which necessitates the use of a fan causing a disturbing noise, which is troublesome, particularly during lectures.